Shock attenuators are employed in ultra-high pressure waterjet cutting systems to smooth the unwanted pressure excursions caused by the reciprocating operation of the system's intensifier. These systems are typically operated to pressures in the 90,000 PSI (620,528 kPa) range, with the typical need for a minimum volume of 1.5 liters but a preferred volume of 2 liters to optimize the attenuator's effectiveness. As used herein, the term “waterjet cutting system” shall include abrasivejet cutting systems wherein abrasive is entrained into the waterjet.
The most economical design has been to use a heavy walled tubular container with a typical inside diameter of 2.375 inches (6 cm) and wetted area of 29 inches (187 cm) or greater. These parameters yield a 2 liter volume large enough to support a dual intensifier system with up to 100 horsepower (74.5 KW) input.
A cylinder with 2.375 inch (6 cm) diameter bore creates approximately 266,000 pounds of force (120 655.57 kg of force) against whatever objects are used to close the ends. This presents a twofold problem: first, the strength requirements due to the straight force loading the closure system and, second, the fatigue loading due to the cycling of the attenuator system, typically 10 to 50 times a day.
Existing attenuator systems employ a variety of mechanical closure methods: screw-in end plugs, end plugs held in with transverse retainer pins, multiple studs anchored in the attenuator body, and tie rods extending the length of the attenuator body. At the very high working pressures to which these attenuator systems are subjected, failure of the closures can have devastating consequences.
The use of screw-in type closure plugs present design problems which, to overcome, are expensive to manufacture. Improper designs can result, and in fact have resulted, in catastrophic failures of the closure system. An example of a catastrophic failure is where the seal material sealing the closure member to the attenuator body migrates into the thread area, increasing the load on the threads due to the thread's resulting maximum diameter being greater than the original (typically, 2.375 inch (6 cm)) seal diameter. A number of such failures are known to have occurred in the waterjet industry. While these failures have fortunately not resulted in any personnel injuries known to the inventor, structural damage has occurred due, for example, to the impact of the end closure parts with structures such as steel I-beams.
The use of six or eight high-strength studs or bolts instead of screw-in type closure plugs has proven to be the most cost-effective configuration, and has been less prone to catastrophic failure. However, there have been failures that resulted, in one case known to the inventor, in the end closure penetrating a twelve meter high roof. Such closures demonstrate through calculations that they can withstand the pressure loading. However fatigue problems, assembly problems and loosening of nuts or bolts can result in the over-loading of two or three of the studs. This damages the studs and causes stress levels in excess of the design limits, creating the possibility of catastrophic failure.
The systems can be complex or simple like the multiple bolt type, but they all have potential for catastrophic failure, including failures due to human error. Accordingly, there is a long-felt need for attenuators with closures that are highly reliable, cost-effective to manufacture and which minimize the chance for human error during assembly.